Comparative investigation of sequentially extracted phosphorus fractions in a sandy loam soil and a swine manure

Sequential fractionation is one of the most common methods used to investigate phosphorus (P) forms in soils. The strategy can be used for evaluating bioavailability of soil P and for investigating the relationship between soil P transformation and soil development. Recently, the strategy of sequential fractionation has been used to investigate manure and compost P and their changes after application to soils. However, the physico-chemical characteristics of animal manure may differ from those of soils. Evaluation is therefore needed to determine if sequentially extracted P forms based on soil studies are applicable for manure. In this study we fractionated P in a sandy loam soil and a swine (Sus scrofa) manure with H2O, 0.5 M NaHCO3, 0.1 M NaOH, and 1.0 M HCl. The P distribution in soil was 0.2% H2O-extractable, 11% NaHCO3-extractable, 58% NaOH-extractable, 14% HCl-extractable, and 16% residual P. In contrast, P distribution in swine manure was 48% H2O-extractable, 19% NaHCO3-extractable, 18% NaOH-extractable, 11% HCl-extractable, and 3% residual P. Elemental analyses, ultraviolet (UV)/visible spectra, and Fourier-transform infrared (FT/IR) spectra revealed distinct differences in chemical composition between soil and swine manure. The NaOH fraction of soil contained 128 mmol aluminum (Al) and 5.8 mmol iron (Fe) per kg of dry matter; however, the NaOH fraction of manure contained only 8.8 mmol Al and 0.6 mmol Fe per kg of dry matter. Concentrations of calcium (Ca) and magnesium (Mg) in various fractions of manure, however, were much higher than in soil. The soil was inorganic mineral-based, and the animal manure was organic residue-based. These data indicate it may not be appropriate to apply soil based fractionation interpretations to swine manure by exclusively assigning NaOH-extractable inorganic P (P-i) to Al- and Fe-P, and HCI-extractable-P-i to Ca-P. We attribute the distinctly different P distribution patterns observed with sequential fractionation of soil and manure to their different physico-chemical properties. These differences must be recognized when developing and interpreting fractionation procedures for manure.